Disc drive machines record and reproduce information stored on concentric circles or spiral tracks on magnetic or optical discs. Data is stored on tracks and is written and read by magnetic heads which must be accurately positioned over one of the tracks on the surface of the disc. The head or transducer, is supported by a flexure to fly over the surface of the disc, the flexure being incorporated in an actuator which responds to signals from a host computer to position the transducer over a selected track. Both linear and rotary actuators have been used for this purpose, but rotary actuators are especially useful in disc drives with small physical dimensions.
A common form of rotary actuator is a V-shaped dual arm assembly. One arm includes a head arm, flexure, and read/write transducer extending out from a pivot toward the tracks on the disc. Extending toward the pivot to form the V-shaped arm assembly is an actuator arm which is coupled to an actuator motor. Rotation of the motor moves the actuator arm and thereby the entire V-shaped assembly, moving the head from track to track. Such rotary actuator mechanisms require a very stable pivot mechanism to support the actuator arm and head support arm; any instability would produce serious positioning inaccuracies.
The pivot mechanism in prior art rotary actuators generally consists of a shaft supporting the juncture of two arms, i.e., an actuator drive arm coupled to the actuator motor and head arm. The pivot point of the actuator arm and head support arm are typically mounted for rotation about this fixed common pivot point. However, assembly of this pivot mechanism is expensive and time consuming to accomplish with proper alignment.
Another problem is that the actuator arm is fit tightly between the fixed pivot point and the pinion which is fit on the end of the actuator motor shaft to drive the actuator arm. This pinion, although designed to be perfectly round, always contains some tolerances which make it oval or otherwise not perfectly circular. This non-circularity causes the actuator arm to be pinched between the fixed pivot point and the pinion. The non-circularity also produces extraneous slack between the gear section of the actuator arm and the pinion, leading to premature wear of the gear section. Both of these conditions, i.e., pinching and slack, have deleterious effects on the actuator arm assembly by inducing undesired stress and wear. The effects of a non-circular pinion are generally referred to as runout.
Attempts to compensate for the effects of runout include those of U.S. Pat. No. 4,845,579 for a Disc Drive Incorporating Automatic Wear Compensation for a Pivoted Arm, by Richard A. Wilkinson, Jr., issued Jul. 4, 1989. In this configuration, the actuator arm and drive motor pinion are biased into engagement by a spring force exerted directly along the axis of the elongated actuator arm. Although this configuration relieves some of the problems induced by a rigidly fixed pivot point, the arrangement is still plagued by the effects of runout and is hampered by friction and wear of its plurality of parts. Furthermore, the configuration is more susceptible to the effects of differential thermal expansion.
A second prior art attempt has been to provide a leaf spring on one side of the pivot point, generally perpendicular to the longitudinal axis of the actuator arm.
There are also several undesirable aspects induced by the leaf spring mechanism. The tolerance of the spring steel used to create the leaf spring is very hard to maintain within critical limitations, as a result, the preload of the spring on the arm varies. Furthermore, since the leaf spring contains a certain amount of twist, a cantilever spring is required to keep the actuator arm from rising vertically. Moreover, mounting the leaf spring to the pivot mechanism requires high precision spot welding which is both time consuming and expensive.